TECTONOSTRATIGRAPHIC HISTORY OF THE UPPER FRANCISCAN (SUBDUCTION ACCRETIONARY ) COMPLEX IN NORTHERN CALIFORNIA: ISSUES OF TERRANE DESIGNATION AND MAPPING

Named units of the NE Diablo Range (NEDR) Franciscan Complex include coherent units, broken and dismembered formations, and various types of mélanges. Sedimentation on the seafloor of the drifting/subducting pre-Farallon (?) Plate and in the associated trench, followed both by subduction zone deformation and metamorphism and by later uplift, produced a distinct tectonostratigraphic sequence. Cretaceous Franciscan sedimentation in the eastern Diablo Range and NEDR (102 to 70 m.y –Joesten et al., 2004; Ernst, 2009) was followed in the NEDR by blueschist facies (BSF) metamorphism between 85 and 45 m.y. (Raymond et al., 1981) and uplift, perhaps between 40 and 30 m.y. (Dumitru, 1989). Yolla Bolly Terrane (YBT) type-area sedimentation was somewhat earlier (120 to 95 my – Dumitru, 2012; Ernst et al., 2012).

Similar tectonostratigraphic sequences with mappable units crop out below the Coast Range Ophiolite (CRO) and other serpentinite/peridotite (sp) units in the NEDR and Tiburon, Alpine Lake, Jenner Headlands, and YBT-type areas (Blake et al., 1982; Bero, 2010; 2013; Prohoroff et al., 2012; Wakabyashi, 1992; 2011). Downward, units include high-grade schist-bearing mélange +/- overlying schist units, foliated metawacke + metachert units, and broken and dismembered formational units. Most sub-CRO mélange and some mélange units have been assigned to the YBT. Structurally lower mélanges have been assigned to the Central Belt Terrane (CBT). Stratigraphies of “YBT” rocks vary regionally. Detrital zircon “YBT” ages cover a 50 my age range, but many YBT characteristics overlap those of the CBT. The YBT rubric seems to have been assigned primarily to metawacke-metachert and associated rock units of blueschist facies grade. Does the use of the terrane rubric as a proxy for detailed structural and stratigraphic mapping provide us with an adequate understanding of the accretionary complex architecture? Probably not. More detailed mapping and detrital zircon ages are needed.